B75-20 Folliculin Deficiency in Monocyte-lineage Cells Exacerbates Influenza Virus Induced Lung Injury and Impairs Post-infectious Repair

Background In chronic lung diseases, viral infections often cause severe exacerbations. We previously reported that Folliculin (FLCN) is downregulated in patients with severe COPD. Mutations in FLCN also underlie Birt-Hogg-Dubé syndrome, which presents with multiple lung cysts. Animal studies have shown that Flcn deletion in lung epithelial cells induces spontaneous emphysematous changes, indicating its essential role in maintaining alveolar integrity. Furthermore, loss of Flcn in monocyte-lineage cells leads to monocyte dysfunction and autoimmune disorders in mice, suggesting that FLCN is critical for maintaining appropriate immune responses against viral infections in the lung. We therefore hypothesized that FLCN in monocyte-lineage cells plays a key role in preventing influenza virus-induced exacerbation and evaluated this using a mouse model. Methods Monocyte-lineage-specific Flcn-deficient mice were generated by crossing Flcnf/f mice with Lyz2-Cre mice (Flcnf/f: control, Flcnf/fLyz2-Cre: Flcn Lyz2 KO). Eight- to ten-week-old mice were intranasally administered influenza virus (H1N1) (PR8) (15 PFU/mouse). Body weight (BW) changes were monitored daily. Twenty-eight days post-infection, lung function was assessed using FlexiVent, and histopathological alterations were examined by hematoxylin-eosin (HE) and Picrosirius red staining. The distribution of TP63-positive regenerating epithelial cells was evaluated by immunostaining. Results Although no histopathological differences were observed between the lungs of control and Flcn Lyz2 KO mice at 8 weeks of age, Flcn Lyz2 KO mice exhibited more severe early-phase BW loss and delayed recovery compared with control mice following influenza infection. Flcn Lyz2 KO mice exhibited more severe early-phase BW loss and delayed recovery compared with control mice following influenza infection. Histologically, Flcn Lyz2 KO lungs displayed expanded abnormal repair areas with fibrotic remodeling, accompanied by distorted alveolar architecture and excessive matrix deposition relative to controls (Fig. 1,2). Consistently, Flcn Lyz2 KO mice showed significantly decreased static lung compliance and increased elastance, indicating impaired tissue recovery. Immunostaining revealed a marked accumulation of TP63-positive cells in the abnormal repair regions of Flcn Lyz2 KO lungs (Fig. 3), suggesting abnormal epithelial regeneration associated with impaired inflammatory resolution after viral injury. Conclusion Flcn deficiency in monocyte-lineage cells may hinder the proper resolution of inflammation and coordinated epithelial repair, leading to excessive or maladaptive lung remodeling following influenza virus infection. These findings suggest that Flcn-dependent monocyte function is indispensable for regulating post-viral recovery and maintaining immune-epithelial balance. Therefore, reduced FLCN expression may contribute to increased vulnerability to infection-related exacerbations and delayed tissue repair after viral injury. This abstract is funded by: VA merit review grant (BX006096)